Monochromators are well known to the prior art. Their function is to isolate selected wavelengths from a source of illumination. This may be done in the form of a scan through the spectrum of interest. In absorption photometers, the amount of absorption by a sample at different wavelengths may serve to identify the sample and its concentration. In applications such as detectors for liquid chromatographs, scanning may be replaced by observation of absorption at one or more specific wavelengths. In either application it is important to be able to accurately select the wavelengths to be observed.
A monochromator normally includes a housing having a radiation entrance slit and a radiation exit slit. Radiation from a suitable source, such as a lamp, is focused on the entrance slit. A collimating mirror is mounted within the housing with its focal point between the entrance and exit slits. It collimates the received radiation and the collimated radiation is caused to fall upon a dispersing element such as an optical grating. The grating disperses the radiation in accordance with its wavelength and a very narrow wavelength range is refocused by the collimator onto the exit slit of the monochromator.
The grating is mounted so that it may be precisely rotated and the degree of rotation is correlated with the output wavelength of the monochromator. In some prior art applications, this is obtained by means of a sine bar mechanism. In such a construction, a lead screw is caused to advance or retract a bar which serves as a camming surface. A pivoted drive arm is secured at its pivoted end to the grating and has at its other end a cam follower which bears against the camming surface. As will be explained more fully below, the amount of movement of the camming surface, and thus the number of turns made by the lead screw, is proportional to the sine of the angle through which the grating rotates. A disadvantage of such a construction as known to the prior art is that the camming surface is relatively long and, when the follower approaches either extreme end of the camming surface, it creates undesirable torque effects which must be overcome.
Filters are commonly used in conjunction with grating monochromators. This is because gratings, unlike prisms, produce higher orders of diffracted radiation which must be suppressed. This is accomplished by inserting filters into the radiation path, usually at the entrance or the exit slit. However, as the grating rotates, different filters are required for different wavelength ranges. For optimum results filters must be changed rapidly and accurately with change of wavelength.
Another problem associated with grating monochromators is that the grating drive must be very accurate. However, it is not a simple matter to obtain lead screws which are accurate to the required degree. Slight variations in screw straightness or roundness and other variations arising during the threading process will often introduce undesirable errors requiring compensation.
Accordingly, it is a primary object of the present invention to provide a simplified and relatively inexpensive filter-grating monochromator which retains the accuracy of more expensive devices. Other objects are to provide such a monochromator having an improved grating mounting, having an improved grating drive mechanism, having a lead screw mechanism which damps out undesirable variations in travel, and having an improved filter change mechanism. Other objects, features and advantages will become apparent from the following description and appended claims.